Rendered image disappeared when I re-opened my project
See this thread: How to save the render result within Blender

Blender does not store renders in the .blend files.  They are stored in RAM until you exit the program, at which time the OS deallocates all applicable memory.  If you want to store the image, you must save it yourself by pressing F3 In the UV editor window.  You can set the options beforehand in the Properties menu:



Or in the Save window: